1. Field of the Invention
The present invention relates to magnetic separation, concentration and other biotechnology applications involving holding, concentration, manipulation or separation of magnetic or magnetizable molecular structures and targets.
2. Background of the Related Art
There are two common types of magnet materials: permanent magnets and ferromagnetic materials. The following is brief background on ferromagnetic and permanent magnetic materials and their use in hybrid magnets.
Permanent Magnets
Permanent magnets are anisotropic or “oriented” materials which have a preferred magnetization axis. When they are magnetized they produce magnetic fields that are always “on” (e.g. they will stick to your refrigerator). The distribution of these fields is dependent upon the “orientation” of the material, its geometry and other material properties. Permanent magnetic material should be distinguished from paramagnetic materials, which are magnetic materials, such as aluminum, that exhibit no magnetic properties in the absence of a magnetic field. Permanent magnets consist of both paramagnetic components, e.g., samarium, neodymium, and ferromagnetic components, e.g., iron, cobalt. During fabrication a crystalline domain structure is created which exhibits spontaneous oriented intra-domain magnetization known as magneto-crystalline anisotropy. This anisotropy is the mechanism that produces strong fields in current rare-earth permanent magnets.
Proprietary processes involving compression of finely pulverized component particles in a strong, ambient magnetic field, sintering of the compressed material and finally remagnetization in a second strong ambient field are used to produce these materials. Once magnetized, these materials will keep these fields indefinitely. However, damage by heating will reduce or eliminate the magnetism.
Soft Ferromagnetic Materials
Soft ferromagnetic materials are macroscopically isotropic or non-oriented. When they have not been exposed to an external magnetic field they produce no magnetic field of their own. These materials include pure iron, common low-carbon steel alloys and more exotic materials such as vanadium permendur which is composed of iron, cobalt and vanadium. The importance of these materials is that they will tend to concentrate and redirect magnetic flux from other sources such as electromagnetic coils or permanent magnets.
Soft ferromagnetic materials typically have some component of iron or other transition metals and include pure iron or alloys of steel. For example, steel that does not evidence magnetism is a macroscopically isotropic material, i.e., has no intrinsic orientation in an annealed state, and is a magnetically malleable material. When exposed to a magnetic field from another source, soft ferromagnetic materials will tend to concentrate and make the field stronger and redirect the field.
Ferrimagnetic Materials
Ferrimagnetic materials are macroscopically similar to ferromagnetic materials but microscopically, ferrimagnetic materials exhibit an anti-parallel alignment of unequal atomic moments. The imbalance in moments is caused by the presence of Fe ions with different oxidation states. This results in a non-zero net magnetization. The magnetic response to an external magnetic field is therefore large but smaller than that for a ferromagnetic material. Thus this material exhibits susceptibility to an applied external field but when the external field is removed, no appreciable remnant field exists in the material because of the weak nature of the magnetic moments of the coupled atoms.
Hybrid Magnets
Hybrid magnets use both permanent magnets and soft ferromagnetic materials. A comprehensive theory of hybrid structures was formulated by Dr. Klaus Halbach for accelerator applications. Combining permanent and soft ferromagnetic materials to form a hybrid magnet became a well-known method in the free electron laser and particle accelerator community, fields unrelated to the present field of use. Such hybrid magnet configurations are used in insertion devices, such as undulators and wigglers, which are used in accelerators that produce high-energy particle beams. Typically very large and powerful magnets are used to accelerate and/or influence particle behavior, causing particles that are exposed to the magnetic fields to “wiggle” or “undulate.” This transverse motion is caused by the Lorentz force effect. See Halbach, U.S. Pat. No. 4,761,584, which discloses a “Strong permanent magnet-assisted electromagnetic undulator” and Halbach, U.S. H450, which discloses a “Magnetic field adjustment structure and method for a tapered wiggler.”
The field gradient structure is created by the combination of linear permanent magnets and specially shaped soft ferromagnetic steel poles. The gradient distributions of these hybrid structures can be controlled and shaped to produce both vertical and horizontal fine-scaled gradients. The forces on magnetic materials are created by these gradients in the field produced by these hybrid structures.
The typical insertion device has magnets arranged in two opposed rows. Each row alternates soft ferromagnetic pole pieces with blocks of permanent magnet material. The magnetic fields of each block of permanent magnet material are oriented orthogonal to the magnetic field orientation of the soft ferromagnetic poles and in the opposite direction of the next block of permanent magnet material. A particle beam is passed along the rows in the space between the two opposing rows. The alternating magnetic orientations along the direction of travel of the particle beam produce precise periodic magnetic fields and cause the particle beam to follow a periodic path or an undulating orbit.
The soft ferromagnetic poles, sometimes referred to as steel poles, can be made from a variety of materials, ranging from exotic materials such as vanadium permendur, which result in better and higher performance magnets, to cheaper materials such as low-carbon steel. Examples of permanent magnet are rare-earth cobalt magnets, such as SmCo magnets, and Neodymium Iron and Boron (NdFeB) magnets.
The permanent magnets act as magnetic flux generators and the soft ferromagnetic poles act as concentrators to produce higher fields with distributions that are more easily controlled. This is called an “iron-dominated” system, i.e., the field distributions in the regions of interest are primarily controlled by the soft ferromagnetic pole geometry and material characteristics rather than the permanent magnets.
Use of Magnetic Devices in Biological Applications
The high performance hybrid magnetic structure herein described relates generally to apparatus and methods for biotechnology applications involving holding, concentration, manipulation or separation of magnetizable molecular structures and targets. The use of magnets in the biological applications involving such techniques as purifying and concentrating molecular particles, separation and concentration of specific targets and ligands for identification of biological pathogens and other molecular particles, has become increasingly popular and widely used. This technique typically involves the immobilization or attachment of the target or structure in a mixture to a magnetic bead. The beads are then separated from the mixture by exposure to a magnetic field. After the structures and targets are released from the beads, the structures and targets can then be used for further applications, testing or identification.
The magnetic beads or particles are, or typically contain, ferrimagnetic material. Magnetic beads may range in diameter from 50 nm (colloidal “ferrofluids”) to several microns. The magnetic beads used in some molecular separation systems contain iron-oxide materials which are examples of ferrimagnetic materials. These beads experience a force in a gradient field but do not retain a remnant magnetic field upon removal of the external gradient field and thus are not attracted to each other. This mechanism allows the beads to disperse in solution in the absence of a magnetic field, but be attracted to each other in the presence of a magnetic field.
Many companies, including Dynal, have developed biological (e.g. antibody-, carboxylate-, or streptavidin-coated) and chemically activated (e.g. Tosyl group or amino group) magnetic particles to aid researchers in developing novel approaches to assay, identify, separate or purify biological particles from heterogeneous or homogenous solutions.
Hybrid magnetic technology has been widely known and used in the accelerator community, however, it has not been applied to any biotechnology application thus far. Commercial methods of magnetic separation, currently in industry use, have been “permanent magnet dominated” systems. This means that the field distributions are controlled by the geometry and orientations of the permanent magnets that are in the plates. Previous technology produces weak fields and gradients that give poorer results and long separation times.
In some cases the current usage of soft ferromagnetic materials is mainly as a magnetic shield, rarely as a means of concentrating the magnetic field. Howe et al., U.S. Pat. No. 5,458,785, disclose a magnetic separation method using a device which incorporates ferromagnetic material as a base and as a field concentrator plate overlying the permanent magnet material that are of alternating magnetic orientation. The differences are readily apparent when cross-sections of the two magnetic structures are compared. The fundamental magnetic circuits of the two structures are different. The design as shown by Howe et al. is limited in terms of field increases from vertical scaling. Any change in the dimensions of each component of the structure vertically or horizontally, changes the field in the region of interest. Furthermore, the fundamental design of the Howe magnetic structure is not capable of producing the level of field strength that can be produced by the current invention.
Li et al. disclose in U.S. Pat. No. 4,988,618, a magnetic separation device using rare earth cobalt magnets spaced equidistant surrounding the wells in a 96-well plate. All the permanent magnets are oriented coplanar to the base and are either uni-directionally or in alternate directions from the next permanent magnet. Yu, in U.S. Pat. No. 5,779,907, discloses a similar apparatus wherein the magnets are positioned in the spaces between the wells of the microplate. Chen et al., in U.S. Pat. No. 6,036,857, disclose an apparatus for continuous magnetic separation of components from a mixture, wherein the magnets are arranged in alternating magnetic orientations, either aligned side-by-side or alternatively slightly offset from each other magnet.
Manufacturers and Suppliers of Magnetic Plates and Separation Devices or Kits
A majority of the magnet plates that are commercially available are made to be used in conjunction with industry standard microtiter plates. The following are examples of major manufacturers and suppliers of magnetic plates and separations devices or kits.
Agencourt Bioscience Corporation (Beverly, Mass.) produces two types of magnetic plates. Available are a 96-magnet plate having ring-shaped permanent magnets and a 96-magnet plate having disc-shaped permanent magnets. The ring-shaped magnets are of the right dimension to allow the wells of a 96-well microtiter plate to fit inside the ring, encircled by the magnet. Magnet plates having ring-shaped permanent magnets are widely used because they are readily available from manufacturers such as Atlantic Industrial Mottels (20 Tioga Way, Marblehead, Mass.) which produces a 96-well “donut” magnet plate. The availability and low cost of these magnets also make assembly of a magnet plate fairly easy and at low cost to the user.
The magnet plate available from Promega, Inc. (Madison, Wis.) uses 24 paramagnetic pins to draw silica magnetic particles (See U.S. Pat. No. 6,027,945 which discloses this method) to the sides of the wells in a thermal cycling plate. An aluminum holder that centers the magnet plate in a robotic platform is also available. A similar pin magnet is also available.
PROLINX, Inc. (Bothell, Wash.) also produces magnetic plates having bar magnets for use with 96-well and 384-well microtiter plates. These magnetic plates hold strips or rectangular block-shaped strong permanent magnets which are placed lengthwise to exert a field on the columns of 96- or 384-well microtiter plates.
Dynal Biotech (Lake Success, N.Y.), which also produces super paramagnetic particles, makes several magnetic plates for use with microcentrifuge tubes and 96-well microtiter plates. Their magnetic plates are made from disinfectant proof polyacetate equipped with rare earth Neodymium-Iron-Boron permanent magnets.